Initial Fluid Fraction in VOF Method Simulations

If you want to simulate the flow behavior of two fluids using the VOF method, you must perform two general steps:

1. Define a VOF section that specifies the two fluids involved. The VOF section defines these fluids as the reference fluid and the secondary fluid for the simulation. You typically define all of the VOF-related material behaviors in the reference fluid definition.
2. Define an initial condition that specifies the relative volume fraction of the two fluids in the VOF section at the start of the simulation. Volume fraction is equal to the volume of reference fluid material contained in a mesh element divided by the overall volume of the element.

In practice the VOF method assumes that the two fluids are immiscible, so you should not specify intermediate values for the two fluids in the initial conditions. Instead, specify that the reference fluid has an initial fluid fraction of 1 and that the secondary fluid has an initial fraction of 0.

The VOF method is based on a diffused interface approach in which the fluid fraction is smeared over the interface across several cells. The fluid fraction varies smoothly from the reference material value of 1 to the secondary material value of 0. Therefore, intermediate values between 1 and 0 are available in the simulation results.